Interpretive Summary: Some phenolic compounds produced by plants, can affect the activity or population characteristics of soil microorganisms. In this study we examined how three phenolic compounds, of increasing complexity, were degraded by soil microorganisms and affected their abundance and diversity in the soil, compared to a glucose control. We measured microbial respiration (activity) and population diversity and size and focused on a class of microorganisms related to nitrogen cycling (ammonia oxidizers). Soil microorganisms metabolized the simplest compound more efficiently than complex compounds but not as efficiently as glucose. While none of the treatment compounds resulted in larger population size, the simple and intermediate complexity compounds resulted in genetic or physiological changes to soil microbial populations compared to the control. However, the compounds did not affect ammonia oxidizers. These studies indicate plant-derived phenolic compounds do influence the characteristics and activity of soil microorganisms but suggest additional studies are needed using a broader range of compounds and a soils and conditions to elucidate the role of polyphenols in determining soil microbiological diversity.

Technical Abstract:
High molecular weight polyphenols (tannins) that leach into the soil may affect microbial populations, for example by serving as substrates for microbial respiration or by selecting for certain microbes . In this study we examined how three phenolic compounds that represent environmentally widespread tannins or their constituent functional groups were respired by soil microorganisms and how the compounds affected the abundance and diversity of soil microorganisms, including ammonia oxidizers. Soil was incubated for two weeks with the monomeric phenol methyl gallate, the small polyphenol epigallocatechin gallate, or the large polyphenol oenothein B. Respiration of the polyphenols during the incubation was measured using the Microresp™ system. After incubation, metabolic diversity was determined by community level physiological profiling (CLPP), and genetic diversity was determined using denaturing gradient gel electrophoresis (DGGE) analysis on DNA extracted from the soil samples. Total microbial populations and ammonia-oxidizing populations were measured using quantitative real time polymerase chain reaction (qPCR). Methyl gallate was respired more efficiently than the higher molecular weight tannins but not as efficiently as glucose. Methyl gallate and epigallocatechin gallate selected for genetically or physiologically unique populations compared to glucose. None of the polyphenols supported microbial growth, and none of the polyphenols affected ammonia-oxidizing bacterial populations or ammonia-oxidizing archaea. Additional studies on the effects of polyphenols on soil microbial populations using both a wider range of polyphenols and a wider range of soils and environments are needed to elucidate the role of polyphenols in determining soil microbiological diversity.